This invention relates to the separation of a gas and a condensable vapor present in the gas.
The problem of the separation of a hot gas which contains a condensable vapor, such as, for example, water vapor-containing air, arises in numerous instances. These instances include the drying of air during recycling in an industrial drying process, or the purification of a gas produced during a high temperature process which is loaded with water vapor. Such is particularly the case in the production of hydrogen by means of electrolysis at high temperature.
In the conventional process for the production of hydrogen, the electrolyzing cells normally operate at a temperature of about 80.degree. C. so that the partial pressure of the water vapor, produced by the contact of hydrogen with the electrolytic bath, is relatively low. To separate this vapor from the hydrogen, it is sufficient to pass the gas over a refrigerating beam. Under such conditions, the quantity of lost heat, corresponding to the latent heat of vaporization of the water, is not significant enough to justify the implementation of means to recover it.
In order to increase the output of the electrolytic cells and to reduce the investment cost per kilogram of hydrogen produced, the production of hydrogen at a temperature in the 200.degree. C. range and at a pressure in the 30 bars range is now being considered. The partial pressure of the water vapor, due to the presence of the concentrated solution of electrolyte, is accordingly around 10 bars. The lower partial pressure is due to the concentration in electrolyte being about 6 bars. The loss of the latent heat of this water vapor, produced at the same time as the hydrogen, could significantly lower the output of the electrolytic cells if the major portion of the latent heat were not recovered by means of an appropriate thermodynamic process during the separation process.
Methods and devices for an optimum implementation of some thermodynamic process are known. Such means and devices have been described, for example, in the French Pat. Nos. 75 114 38 (published under Nr. 2307227), 76 14965 (published under Nr. 23 52 247), and 77 07041. These patents pertain to systems which will hereafter be called "polytropic machines," and are herein incorporated by reference.
These machines consist of a series of cells having stepped pressures/temperatures, in which a working fluid present in each cell circulates as a saturating vapor in contact with its liquid. In addition, there exists, at least in some cells, one or more heating or cooling beams connecting the cells with temperature-bearing fluids which bring heat from a heat generating source or extract the heat present therein for use elsewhere. Finally, each cell is linked to the next cell or cells, depending on whether the primary heat entering the process is available, on the average, at a low or high temperature level, whether the vapor ascends or descends different levels of pressure/temperature, and, on the path of the liquid, which circulates in a direction opposite to that of the vapor, and in even quantity through an opening gauged to descend the levels of pressure/temperature, or through a pump to ascend them. In order to be familiar with the structure and operation of such devices, the artisan need only review the disclosure of the above-mentioned patents.
In cases where the temperature-bearing fluid transports heat (it circulates through a series of steps in the direction of the decreasing temperatures), vapor of the working fluid is produced by the boiling of the liquid contained in the cell, and, accordingly, vapor from the working fluid condenses. Thus, the output of vapor and liquid evolves from step to step according to the quantity of heat added or subtracted, in compliance with the law Q(T), according to which the addition or subtraction of heat is achieved; that is to say, as a function of the size of the exchange beams on a practical basis.
It must be noted that at the interface of two successive cells, the total amount of working fluid entering as a vapor or liquid is always equal to the sum of the working fluid coming out as liquid or vapor, with the output of vapor or liquid circulating in the opposite direction being always equal.
It should also be noted that the polytropic machines described in the above-mentioned patents could consist of a simple elementary series, four of which are as follows, to wit:
(1) Cooling off series with compressors, used for a "condensation process with absorbed work."
(2) Heating up series with compressors, used for a "boiling process with absorbed work."
(3) Cooling off series with turbines, used for a "condensation process with provided work."
(4) Heating up series with turbines, used for a "boiling process with provided work."
These four elementary series all include a step including an open extremity through which the outputs of liquid and vapor of the working fluid enter and exit, and a step of closed extremity where the working fluid is either entirely vaporized or entirely condensed.
The following Table describes the side where the open step is located, the entrances and exits of the working fluid in relation to the series under consideration, as well as the direction of the temperature-bearing fluid:
______________________________________ Direction of Type of Open Working Temperature Series Step Fluid Bearing Fluid ______________________________________ Heating up series higher liquid enters decreasing with compressors temperature vapor exits temperatures Cooling off series lower vapor enters increasing with compressors temperature liquid exits temperatures Heating up series lower liquid enters decreasing with turbines temperature vapor exits temperatures Cooling off series higher vapor enters increasing with turbines temperature liquid exits temperatures ______________________________________
In these systems, the temperature-bearing fluid may go through several successive steps, or one single step. As a limit, we may have a temperature-bearing circuit of the same kind per step of a specific series.